Wheel speed sensor for drive axle

ABSTRACT

A wheel speed sensor for a wheel mounted on the end of an axle shaft of a drive axle can be utilized in an anti-wheel lock brake control system. The sensor includes an electromagnetic sensing device which is mounted on the drive axle housing in alignment with the axle shaft inwardly of the wheel. An excitor rotor of the sensor is preferably mounted on the differential casing about the axle shaft in alignment with the sensing device for rotation while maintaining the distance between the sensing device and the rotor within a predetermined limit without allowing contact therebetween. A C-shaped spring partially encircles the rotor as a pair of tabs at its ends extend inwardly through the rotor to make contact with a corresponding pair of grooves in the axle shaft. The C-shaped spring member drives the rotor in response to rotary movement of the axle shaft but allows removal of the axle shaft for maintenance while the rotor is retained within the protective interior of the housing.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a wheel speed sensor for a drive axle and,more specifically, to such a sensor which can be located inwardly of thewheel, for example, in the differential area and may be utilized in ananti-wheel lock brake control system. U.S. Pat. Nos. 3,743,362 and3,790,227 are hereby incorporated by reference.

2. Description of the Prior Art

Modern anti-wheel lock brake control systems operate in response toelectrical signals indicative of the angular velocity of one or more ofthe vehicle wheels. Usually these signals are derived from sensors whichinclude an electromagnetic sensing device having at least one magnet andcarried by a stationary portion of the vehicle such as the axle housingand a toothed or notched metal rotor which rotates in response to wheelrotation and is located opposite the sensing device. As the rotor teethpass the electromagnetic sensing device, the resulting variations influx produce a voltage, the frequency of which is a function of theangular velocity of the wheel and the number of teeth in the rotor.

To produce the required voltage level of the velocity signal, thesensing device and excitor rotor must be accurately located with respectto each other during initial assembly and must remain in thispreselected relationship even after extended periods of operation inwhich the components are subjected to road shocks and repeatedvibration. It is also essential that installation of the sensor assemblybe accomplished in a manner adaptable to mass production methods tominimize the need for skilled technicians and elaborate installationtechniques. It has heretofore been the general practice to provide wheelspeed sensors for braking systems which are located in the wheel region.It has, however, been found that these sensors have caused some problemsby their location at the wheel since care must be taken to prevent theirdamage during wheel maintenance.

Although, as shown in U.S. Pat. Nos. 3,138,970 and 3,769,533,consideration has been given to wheel speed sensors for drive axleswhich are located in the differential area of the axle, theiracceptability has been limited. In these systems, a rotor is directlymounted to the axle shaft and the electromagnetic sensing device ismounted on the differential housing for extension therein in alignmentwith the rotor.

However, with improved anti-wheel lock brake control systems, betterbraking control and shorter stopping distances have been obtained when ahigher number of teeth are used in the rotor, for example, 60 to 120teeth. Increasing the number of teeth for better resolution of thesystem also requires that the distance between the sensing device andthe rotor be minimized and places greater emphasis on maintaining thisdistance within predetermined limits throughout rotor rotation. This hasbeen found to be significant with an anti-wheel lock brake system suchas disclosed in U.S. Pat. Nos. 3,743,362 and 3,790,227, which areincorporated by reference herein. The system disclosed therein, forexample, is an axle-by-axle system which includes for each axle adigital computer for controlling a high capacity valve which is capableof delivering maximum operating air pressure to both brakes on the axle.The computer logic automatically controls the valve so that it iscapable of delivering only a proportion of the operating air pressure inan effort to keep the level of air pressure just below that at whicheither wheel will lock. The computer must rely on accurate wheel speedindication to be capable of making constant decisions at a rate of about50 times a second on whether to increase or decrease air pressure, ormaintain it at the same level. Because the system is capable ofproportioning the air pressure rather than being limited to simplyturning it fully on or off as was the practice in many systemsheretofore utilized, it produces a smoother, controlled stop in shorterdistances.

Mounting a rotor directly to an axle shaft, as taught by the prior artdiscussed hereinabove, has generally been found for several independentreasons to be incapable of satisfying the requirements of these improvedsystems. Firstly, the normal manufacturing tolerances for an axle shaftand the designed "float" allowed for the side gears of the differentialwill not insure a sufficiently accurate rotor location with respect tothe portion of the drive axle housing on which the sensing device hasbeen mounted. Without redesigning to eliminate the float and/or withoutsignificantly decreasing the tolerances of the axle shaft during itsmanufacture, the portion of the shaft to which the rotor is secured canbe inaccurately aligned with the housing so that the axle shaft willmove transversely within the housing during its rotation.

Additionally, since during initial installation the axle shaft must bephysically inserted into the differential through the interior of thedrive axle housing, potential damage to the rotor mounted thereon ispossible by its accidentally making contact with the interior. Thispotential damage to the rotor during installation also exists each timethe axle shaft must be removed for maintenance purposes.

Further, since the end of the housing adjacent the wheel area has aninside diameter only slightly larger than the outside diameter of theaxle shaft, the size of the rotor is also limited if it is permanentlyand directly mounted thereon thus limiting the number of teeth that canbe utilized for improving system resolution.

Still further, the fact that the axle shaft is sometimes removed formaintenance presents another reason, perhaps as significant as anypresented hereinabove, for not having the rotor permanently mounted onthe axle shaft. When the rotor is no longer protected by the housing andis generally exposed to the environment of a maintenance area, damage tothe surface or teeth of the rotor is a significant possibility whichcould seriously affect the operation of the wheel speed sensor when itis reinstalled.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a wheelspeed sensor for a drive axle which may be located inwardly of the wheelarea, as, for example, in the differential area.

It is another object of the present invention to provide a wheel speedsensor of the type described which is simple to install and which willnot be susceptible to damage during wheel or axle maintenance.

It is a further object of this invention to provide a wheel speed sensorof the type described in which the rotor is mounted for rotation inresponse to rotation of the axle shaft without being directly mountedthereon prior to axle installation.

It is still another object of the invention to provide a wheel speedsensor of the type described in which the misalignment of the axle shaftwill not interfere with efforts to maintain a predetermined spacebetween the rotor and the sensing device.

It is still a further object of the invention to provide an anti-wheellock brake system which utilizes a computer to control a valve toprovide proportional amounts of air pressure to both brakes of a driveaxle to prevent either wheel from locking, wherein the computer receiveswheel speed indication for this purpose from each of a pair of sensorsmounted adjacent the differential mechanism of the drive axle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary sectional view of the preferred embodiment,including various features of the invention.

FIG. 2 is a view as seen along the line 2--2 of FIG. 1.

FIG. 3 is a view as seen along the line 3--3 of FIG. 1.

FIG. 4 is a fragmentary sectional view of an alternative embodiment ofthe invention.

FIG. 5 is a fragmentary sectional view of another alternative embodimentof the invention.

FIG. 6 is a side view of the rotor of the embodiment shown in FIG. 5.

FIG. 7 is a side view of the drive spring of the embodiment shown inFIG. 5.

FIG. 8 is a fragmentary sectional view of yet another alternativeembodiment of the invention.

FIG. 9 is an end view of the drive spring for the embodiment shown inFIG. 8.

FIG. 10 is a fragmentary sectional view of still another alternativeembodiment of the present invention.

FIG. 11 is a fragmentary sectional side view of one more alternativeembodiment of the present invention.

FIG. 12 is an end view of the embodiment shown in FIG. 11 as seen alongline 12--12.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As seen in FIG. 1, a wheel speed sensor 2 includes an electromagneticsensing device 4 and an excitor rotor 6 which are located on a driveaxle 8 of a vehicle. The sensor 2 as shown in the preferred embodimentis located remotely of the wheel (not shown), which would be to theright as viewed, and adjacent a differential mechanism (only partiallyshown), which would be to the left. More specifically, the sensor 2 isshown at an axle shaft 10 in general alignment with a portion 12thereof, which is adjacent the end of the differential casing 14. Therewould, of course, be two sensors, one for each axle shaft and wheel, inan anti-wheel lock brake system like that discussed hereinabove.

The sensing device 4 extends through a cavity 16 of the drive axlehousing 18 where an appropriate retaining means 20 and sealing means 21maintains its location through operation and insures oil within thehousing 18 will not be lost. A key 22 of sensing device 4 is receivedwithin the cavity 16 to insure proper alignment and prevent rotation ofthe sensing device 4. Electrical wiring 23 transmits a signal from thesensing device 4 for use in an anti-skid braking system or any otherspeed indicating system as desired.

The rotor 6 includes a plurality of teeth 24 which are disposed to passby the sensing device 4 so that it may generate the signal indicative ofspeed. The rotor 6 is mounted for this purpose of a sleeve member 26which is in turn mounted within a cavity 28 of the differential casing14. The cavity 28 is adapted to receive the axle shaft 10 so that thesleeve member 26 and rotor 6 will generally rotate about the axle shaft10. The rotor 6 is maintained axially on the sleeve member 26 by aninward end 30 making contact with the differential casing 14 and theoutward end 32 resting against a snap ring 34 which is retained within acircumferential groove 36 of the sleeve member 26. Since thedifferential casing 14 is mounted for rotation within the housing 18 atbearings 38 held in position by an adjusting nut 40, its general axis ofrotation A will be relatively fixed with respect to the housing 18.Accordingly, the axis of rotation of the rotor 6 will be disposed withrespect to the rigidly mounted sensing device 4 to maintain apredetermined distance therebetween. During the manufacturing of thevarious elements discussed hereinabove, the general tolerances normallymaintained will result in a predetermined distance between the sensingdevice 4 and the rotor 6 which is within the desired operating limits toinsure accurate speed indication.

Referring additionally to FIGS. 2 and 3, the preferred rotor 6 includes60 teeth 24 which are to pass by a pair of sensing elements 42. The gapG between the teeth 24 and the elements 42 should be about .030 inchesand no greater than .040 inches as the rotor 6 rotates about the axis ofrotation A of the differential casing. However, the generalmanufacturing tolerances of the axle shaft and the general tolerancesbetween the side gear (not shown) and the pinion gear (not shown) of thedifferential (not shown) will not insure that a gap of less than 0.040inches can be maintained if a rotor is mounted directly on the axleshaft. The axle shaft may be sufficiently misaligned during operation totransversely move toward and away from the housing to either undesirablyalter the speed signal or to cause destructive contact between the rotorand the sensing device. However, the means of mounting disclosed hereinfor the present invention prevents either of these occurrences.

To provide a means whereby the rotor 6 will rotate in response to rotarymovement of the axle shaft 10 but will be unaffected by any transversemovement thereof, a flexible drive means of the preferred embodiment isin the form of a C-shaped spring 44. The C-shaped spring 44 partiallyencircles the rotor 6 in a region which is remote from the teeth 24. Thespring 44 includes at its opposite ends inwardly extending tabs 46. Eachtab 46 is adapted to extend through aligned openings 48 and 50 of therotor 6 and sleeve member 26, respectively. The inward ends 52 of thetabs 46 extend into a splined area 54 at the portion 12 of the axleshaft 10. The C-shaped spring 44 is therefore capable of compensatingfor misalignment of the axle shaft 10 with respect to the sleeve member26 and rotor 6 which are themselves, as explained hereinabove, alignedwith the housing 18. With even the most significant transverse movementof the axle shaft 10, both tabs 46 will be maintained within a groove ofthe spined area 54 because of the resilience of the spring 44. It hasbeen found that slight movement of the inward end 52 of the tab 46within the groove of the splined area 54 and slight movement of the tab46 within the openings 48 and 50, which occurs when the axle shaft movesin a direction generally perpendicularly to the tabs 46, might interferewith rotor operation.

Accordingly, the indented portion 56 of the spring 44 has been includedto be snugly received within aligned openings 55 and 57 of the rotor 6and sleeve member 26, respectively. While the indented portion 56 allowsthe spring 44 to move in a direction perpendicularly of the tabs 46, italso restricts any rotation of the spring 44 about the tabs 46 andgenerally prevents relative rotational movement between the rotor 6 andthe axle shaft 10 during operation.

There are a number of features found in the preferred embodiment ofFIGS. 1, 2 and 3 and which may be found in other embodiments discussedhereinbelow which should be discussed for a clearer understanding of theinvention. For example, as mentioned hereinabove, the interior of thehousing 18 is filled with lubricating oil. Accordingly, it has beenfound that the sleeve member 26 has been provided sufficient lubricationto allow its rotation within the cavity 28 of the differential casing14. It is also well known in the bearing art that the proper selectionof compatible surface materials between members or the introduction noflubricating parts or passageways could further insure against wear ifrequired. It should be noted, however, that the relative rotation of thesleeve member 26 with respect to the differential casing 14 only occurswhen the differential mechanism is being utilized to allow one wheel torotate at a different speed than the other. The location of the sleevemember 26 within the differential casing 14 therefore has in thispreferred embodiment been chosen because of the minimum relativerotation therebetween which is expected during normal vehicle operation.

Although the rotor 6 is shown with teeth 24, it should be understoodthat other forms of rotors might be used. A plastic rotor, for example,could be used and might include a smooth outer surface with a pluralityof metal bars encased therein to produce the electromagnetic forcevariations required for a speed signal.

Although a splined area which completely encircles the axle shaft 10 isshown, any reasonable alternative such as a single groove at eachcontact point or a pair of adjacent raised portions might be utilized toproduce the same results.

Another significant feature of the preferred embodiment is betterunderstood by a general explanation of the installation of the axleshaft 10 and its possible removal during maintenance. The drive axlehousing 18 in most installations extends to the wheel area where itclosely encircles the axle shaft 10. When the drive axle 8 is assembled,the differential mechanism is initially mounted within the housing 18.Each axle shaft 10 is then positioned through the small opening at theend of the housing 18 from the wheel end. The inward end of the axleshaft 10, which is splined, is then "threaded" through the opening inthe differential casing 14 to be eventually received within the sidegears of the differential mechanism. It can now be seen that the drivemeans of the preferred embodiment in the form of the C-shaped spring 44accommodates this installation procedure. As the axle shaft 10 is movedaxially past the spring 44, the tabs 46 will expand outwardly under thegeneral biasing of the spring 44. When the axle shaft 10 is fullyinstalled and is initially rotated, the force of spring 44 will insurethat the tabs 46 are engaged with the splined area 54 where it will bethere maintained throughout operation. Similarly, if the axle shaft 10is removed for maintenance reasons, the spring 44 will be expanded toallow axial movement. The axle shaft 10 can then be reinstalled with thespring again being automatically engaged for operation.

As seen in FIG. 4, an anternative wheel speed sensor 58 includes asensing device 4 like that of the previous embodiment and a rotor 60.However, the rotor 60 has been mounted on an extension 62 of thedifferential casing 64 for rotation thereabout. The rotor 60 includes acircumferential groove 66 in its internal bearing surface 68 and theextension 62 includes a groove 70 aligned with the groove 66 in itssurface 72. The grooves 66 and 70 cooperate to allow a snap ring 74 tobe positioned therein for retention of the rotor 60 on the extension 62.

The rotor 60 includes driving means similar to that discussedhereinabove and has an inwardly extending collar portion 76 whichclosely encircles the spline area 54 of the axle shaft 10 for thispurpose. The collar 76 includes a pair of openings 78 and anotheropening (not shown) which are adapted to respectively receive the tabs46 and the indented portion 56 of a C-spring 44 which has been explainedabove. The rotor 60 of the wheel speed sensor 58 therefore will rotaterelative to the differential casing in a manner similar to the rotor 6of the preferred embodiment without the need of a sleeve member 26.

As seen in FIG. 5, another alternative sensor 80 includes a sensingdevice 4 and a rotor 82. Like the embodiment of FIG. 4, the rotor 82 ismounted on an extended portion 84 of the differential casing 86.Retention of the extended portion 84 is again provided by a snap ring 87acting on grooves 88 and 90 of the rotor 82 and extended portion 84,respectively. However, to provide the flexible drive means between therotor 82 and the axle shaft 10, a spring drive 92 is utilized.

As better seen in FIGS. 6 and 7 of the rotor 82 and the spring drive 92,respectively, as viewed from right in FIG. 5, they cooperate to insurethat the rotor 82 will again respond to rotational movement of the axleshaft 10. The rotor 82 again includes a plurality of teeth at its outercircumference but additionally has three evenly spaced cavities 94 aboutits interior circumference 102. Each cavity 94 is adapted to receive aleg 96 extending inwardly from a base plate 98 of the spring drive 92.Each leg 96 includes a foot 100 which extends into the groove 88 so thatthe rotor 82 will be firmly held between the base plate 98 and the foot100. The location of the leg 96 is outwardly of the interiorcircumference 102 of the rotor 92 to prevent its interference withrotation about the extended portion 84. The spring drive 92 alsoincludes three evenly spaced cantilever spring members 104 which extendinwardly from the base plate 98 at an angle to facilitate installationof the axle shaft 10. Each spring member 104 includes a stamped andformed dimple 106 for engagement with the splined area 54 to preventrelative rotational movement of the spring drive 92 with respect to theaxle shaft 10.

Still another embodiment of the invention is shown by the sensor 110 inFIG. 8. Like the embodiments of FIG. 4 and FIG. 5, a rotor 112 isadapted for rotation about an extended portion 114 of the differentialcasing 116. However, the rotor 112 is retained on the extended portion114 and is driven about the axle shaft 10 by the same retaining anddrive member 118. The rotor 112 again includes equally spaced cavities120 about its interior circumference 122. As better seen in FIG. 9, theretaining and drive member 118 again includes a base plate 124 to abutthe outward end of the rotor 112. A curved leg portion 126 extends fromthe base plate 124 along the cavity 120 toward the inward end of therotor 112. An extended end 128 of the leg portion 126 curves inwardly tobe received within a circumference groove 130 of the extended portion114 of the differential casing 116. With the curved leg portion 126 sopositioned, the rotor 112 is prevented from being axially removed fromthe extended portion 114 as it is entrapped between the base plate 124and the differential casing 116. The curved leg portion 126 within thecavity 120 also insures relative rotation of the rotor 112 with respectto the extended portion 114 without any interference therebetween. Aninwardly extended cantilever spring member 132 similar to the springmember 104 is again utilized to insure that the retaining and drivingmember 118 will rotate in response to axle shaft movement.

Although, in the embodiments discussed hereinabove, rotation of a rotorhas been obtained by its being mounted on or relative to thedifferential casing, an alternative sensor 140 shown in FIG. 10discloses another means for mounting a rotor 142. The rotor 142 ismounted for rotation about an extended portion 144 of an adjusting nut146. Retention of the rotor 142 on the extended portion 144 of anadjusting nut 146. Retention of the rotor 142 on the extended portion144 and rotary driving of the rotor 142 are accomplished with a snapring 87 and a spring drive 92, respectively, as were discussedhereinabove in the embodiment of FIG. 5. The sensor 140 is not generallypreferred to those discussed hereinabove because of the relativerotational movement that will exist during operation between the rotor142 and the extended portion 144. The adjusting nut 146 will be fixed toprevent its rotation while the rotor 142 will follow the movement of theaxle shaft 10. Although in the sensor 140 the relative movements areconsidered to be significantly greater, the general bearing surfacebetween the extended portion 144 and the rotor 142 is similar since itis felt that sufficient lubrication exists within the housing 18 toprevent its failure and that the spring drive 92 operates in a manner toprevent any significant transverse loads being transmitted from the axleshaft 10. However, it should be obvious from this and other embodimentspresented hereinabove that a small bearing means such as a plurality ofneedle bearings might be utilized to further insure satisfactory freerotation of the rotor 142 throughout operation.

As seen in FIGS. 11 and 12, a final alternative embodiment of theinvention is shown in the form of wheel speed sensor 150. The wheelspeed sensor 150 includes a housing 152 adapted to be bolted to theinterior of the drive axle housing 154. A device 4 which is againmounted for extension through the drive axle housing 154 now alsoextends into the sensor housing 152. The sensor housing 152 includes forinstallation of a rotor 160 a cover plate 156. As seen in FIG. 11, withthe cover plate 156 installed by a bolt means 158, the rotor 160 of thesensor 150 is also retained within the sensor housing 152 for accuraterotation relative to the sensing device 4. The rotor 160 includes aninwardly extending collar 162 to facilitate its coupling to the axleshaft 10. The collar 162 includes a pair of openings 164 and an opening172 therethrough to respectively receive the legs 166 and the indentedportion 170 of a C-shaped spring 168 similar to that found in theembodiments of FIG. 1 and FIG. 4 for engagement with a splined area ofthe axle shaft 10.

Although the sensor 150, like those presented hereinabove, is againlocated in the differential mechanism, it can now be seen that a sensormay be provided in accordance with the present invention which might belocated at almost any position along the drive axle housing. It shouldalso be clear that other drive means might be utilized and that otherhousing designs could be employed to respectively insure that the rotorwill rotate in response to axle shaft movement and that the rotor willbe maintained within a fixed distance of the sensing device.

Although the entire discussion provided hereinabove has includedrepeated references to a drive axle, this embodiment provides adequatestructure to demonstrate that a sensor of the present invention might beutilized on other axle configuration. If, for example, there exists anaxle, with or wiithout a differential mechanism, in which the wheel isrigidly mounted to an axle shaft which extends inwardly therefrom androtates therewith, a sensor such as shown in FIGS. 11 and 12 may beadapted to provide wheel speed indication. It could then be mounted onthe housing or frame inwardly of the wheel in alignment with the axleshaft so that it would be advantageously positioned to allow wheel andaxle maintanence without being exposed to damage.

As another point of consideration, it should be noted that each of theembodiments presented hereinabove includes a drive means, in some formor another, which is flexible. This is the preferred configuration tosimplify installation of the axle shaft and to more readily accommodateany axle shaft misalignment, one of the primary problems which thepresent invention is intended to satisfy. It should be recognized thatone skilled in the art might devise a number of alternative flexibledrive means while still falling within the scope of the invention.However, it is also possible that such a means which is not flexiblecould also be employed and be within the scope of the invention. Forexample, the axle shaft could be provided an encircling array ofoutwardly extending teeth and the rotor could be provided a matchingarray of inwardly extending teeth. The teeth would not be closely matingbut would be deliberately designed to float with respect to each otherto prevent the transverse motion of the axle shaft from passing to therotor. The teeth would simply be designed long enough to insure somecontact between the two arrays throughout axle shaft rotation totransfer its rotary motion to the rotor.

A flexible drive means would also not be required if the sensor werelocated along the axle shaft at a position which was not misaligned withthe housing during its rotation. The present invention also teaches thata rotor may be mounted at a location other than permanently and directlyon the axle shaft to allow it to be readily engaged and disengaged fromthe shaft and to thereby allow a larger sized rotor and/or one that isless likely to interfere with wheel or axle maintenance. Without amisaligned axle shaft, the invention still provides an advantage overthe configurations heretofore utilized and could, for example, bepracticed within the scope of the claims by providing a rigid drivemeans such as the matching arrays of teeth discussed above. However, theteeth could, in this case, also be designed to closely mate if there isno longer a need to compensate to transverse movement of the axle shaft.This configuration would still solve some of the problems presentedhereinabove with respect to the prior art since the rotor would beretained within the housing by being mounted to restrict its axialmovement within the housing to thereby prevent its removal as the axleshaft is being removed.

We claim:
 1. A wheel speed sensor for a wheel mounted on an end of anaxle shaft of a vehicle for rotation therewith, said sensorcomprising:an electromagnetic sensing device capable of being mounted ona housing adjacent said axle shaft in general alignment with a portionof said axle shaft inwardly of said wheel; an excitor rotor capable ofbeing mounted generally about said portion of said axle shaft inalignment with said sensing device mounted on said housing for rotationabout an axis which is disposed with respect to said sensing device tomaintain a distance between said sensing device and said rotor within apredetermined limit without allowing contact therebetween; and a drivemeans mounted relative to said rotor and for making contact with saidportion of said axle shaft to cause said rotor to rotate in response torotary movement of said axle shaft.
 2. A wheel speed sensor for a wheelmounted on an end of an axle shaft of a drive axle for a vehicle, saiddrive axle including a differential mechanism for operationally rotatingsaid axle shaft of said wheel, said sensor comprising:an electromagneticsensing device capable of being mounted on a housing of said drive axlein general alignment with a portion of said axle shaft inwardly of saidwheel; an excitor rotor capable of being mounted generally about saidportion of said axle shaft in alignment with said sensing device mountedon said housing for rotation about an axis which is disposed withrespect to said sensing device to maintain a distance between saidsensing device and said rotor within a predetermined limit withoutallowing contact therebetween; and a drive means mounted relative tosaid rotor and for making contact with said portion of said axle shaftto cause said rotor to rotate in response to rotary movement of saidaxle shaft.
 3. A wheel speed sensor as set forth in claim 2 wherein saiddrive means includes structure which facilitates engagement anddisengagement with said axle shaft when said axle shaft is respectivelyinstalled within said housing and removed from within said housing.
 4. Awheel speed sensor as set forth in claim 2 wherein said drive meansincludes a resiliently deflectible portion to facilitate said contactwith said portion of said axle shaft despite transverse movement of saidportion of said axle shaft with respect to said housing during its saidrotary movement.
 5. A wheel speed sensor assembly for a wheel mounted onan end of an axle shaft of a vehicle for rotation therewith, said sensorsystem comprising:an electromagnetic sensing device mounted on saidhousing adjacent said axle shaft in general alignment with a portion ofsaid axle shaft inwardly of said wheel; an excitor rotor mountedgenerally about said portion of said axle shaft in alignment with saidsensing device for rotation about an axis which is disposed with respectto said sensing device to maintain a distance between said sensingdevice and said rotor within a predetermined limit without allowingcontact therebetween; and a drive means mounted relative to said rotorfor making contact with said portion of said axle shaft to cause saidrotor to rotate in response to rotary movement of said axle shaft.
 6. Awheel speed sensor assembly for a wheel mounted on an end of an axleshaft of a drive axle for a vehicle, said drive axle including adifferential mechanism for operationally rotating said axle shaft ofsaid wheel, said sensor system comprising:an electromagnetic sensingdevice mounted on said housing of said drive axle in general alignmentwith a portion of said axle shaft inwardly of said wheel; an excitorrotor mounted generally about said portion of said axle shaft inalignment with said sensing device for rotation about an axis which isdisposed with respect to said sensing device to maintain a distancebetween said sensing device and said rotor within a predetermined limitwithout allowing contact therebetween; and a drive means mountedrelative to said rotor for making contact with said portion of said axleshaft to cause said rotor to rotate in response to rotary movement ofsaid axle shaft.
 7. A wheel speed sensor assembly as set forth in claim6 wherein said drive means includes structure which facilitatesengagement and disengagement with said axle shaft when said axle shaftis respectively installed within said housing and removed from withinsaid housing.
 8. A wheel speed sensor assembly as set forth in claim 6wherein said drive means includes a resiliently deflectible portion tofacilitate said contact with said portion of said axle shaft despitetransverse movement of said portion of said axle shaft with respect tosaid housing during its said rotary movement.
 9. A wheel speed sensorassembly as set forth in claim 8 wherein said drive means includesstructure which facilitates engagement and disengagement with said axleshaft when said axle shaft is respectively installed within said housingand removed from within said housing.
 10. A wheel speed sensor assemblyas set forth in claim 9 wherein said portion of said axle shaft includesa pair of longitudinal grooves at opposite sides of said portion andsaid drive means includes a C-shaped spring having a body portion and apair of inwardly extending tabs at the ends of said body portion whichsaid tabs respectively extend through a pair of openings in said rotorto be respectively received within said grooves of said portion of saidaxle shaft.
 11. A wheel speed sensor assembly as set forth in claim 10,wherein said body portion includes an indented portion which extendsinwardly to be snugly received within another opening in said rotor assaid body portion extends partially around the outer surface of saidrotor said indented portion tending to restrict undesired rotation ofsaid C-shaped spring about said tabs.
 12. A wheel speed sensor as setforth in claim 9, wherein said rotor includes at least two grooves inits interior circumferential surface; said portion of said axle shaftoperationally includes at least two longitudinal grooves which arecircumferentially disposed one from the other; and said drive meansincludes a drive member of resilient material which encircles said axleshaft, is positioned against an end of said rotor, and includes a legadapted to be received within each said groove of said rotor withoutinterferring with its said rotation and a cantilever extension extendinginto each of said longitudinal grooves of said portion of said axleshaft.
 13. A wheel speed sensor assembly as set forth in claim 6 whereinsaid rotor is mounted relative to a differential casing of saiddifferential mechanism.
 14. A wheel speed sensor assembly as set forthin claim 13 wherein said rotor is mounted on an extended portion of saiddifferential casing for rotation thereabout.
 15. A wheel speed sensorassembly as set forth in claim 13 wherein said rotor is mounted on aninternal sleeve member mounted within a cavity of said differentialcasing through which said cavity of said axle shaft extends for rotationof said sleeve member about the interior of said cavity.
 16. A wheelspeed sensor assembly as set forth in claim 6 wherein said rotor ismounted on an extended portion of an adjusting nut for rotationthereabout which said adjusting nut is adapted to retain a bearingmember of said differential casing.
 17. A wheel speed sensor assembly asset forth in claim 6 further including a sensor housing mounted withinsaid housing of said drive axle, wherein said rotor is mounted on saidsensor housing for rotation thereabout.